1. Field of the Invention
The present invention relates to a fault detection circuit for use with a power control device to detect a short circuit in the power control device. Particularly, the fault detection circuit detects a shorted power control device by comparing the power input current and a power output current when the power control device is intended to be off. Further, the present invention uses the output of the power control device itself to open the respective circuit if that power control device is shorted. The present invention is applicable to DC or AC power switching applications. Also, a typical power control device is a solid state power switch, although the present invention is not limited to such as power control device.
2. Description of Related Art
Solid state power switches used as power control devices tend to fail in shorted states, which is often unacceptable in power switching applications. Various fault detection circuits for detecting such short circuits in power switching applications are known. In one such technique, a second solid state power switch device is used to turn off the current when a primary solid state power switch fails to respond. This method is not fail-safe, since both devices may be shorted by one large power surge. In another technique, a mechanical fuse or circuit breaker is added in series with the solid state power switch device. This technique is based on the assumption that when the solid state power switch device is shorted, so is the load. This is not always the case. Therefore, this technique fails to detect a shorted solid state power switch where the power switch is shorted but the load is not.
In yet another technique, a thermal/current fuse is placed in close proximity to a power control device such that the fuse opens the circuit when the solid state power switch carries the full load current, thereby overheating the fuse, or in the event that the temperature of the power switch exceeds the fuse""s temperature rating. This technique is effective even where the power switch is shorted but the load is not. However, this technique is not fail-safe in that it requires the power control device itself to exceed the fuse""s temperature or current rating in order for the circuit to be opened.
The present invention solves the aforementioned drawbacks of the prior art by providing a fault detection circuit that compares the conduction state of a switch coupling power to a load, and if the switch is improperly closed the present invention utilizes the energy delivered to the load to blow a fuse and decouple the load from a power source. In one embodiment, the present invention provides a fault detection circuit comprising an input voltage coupled to an output load through a control switch; a control signal regulating the conduction state of said control switch; and fault detection circuitry receiving said control signal and a signal indicative of the energy delivered to said load, and adapted to determine if said control switch is in the proper conduction state based on said control signal, said fault detection circuit further adapted to decouple said input voltage from said load using said energy delivered to said output load if said control switch is in an improper conduction state.
In method form, the present invention provides a method for fault detection circuit operation, comprising the steps of:
coupling an input voltage to a load through a control switch;
regulating the conduction state of said control switch with a control signal;
determining if said control switch is in a proper conduction state based on said control signal and the energy delivered to said load;
decoupling said input voltage from said load using said energy delivered to said load if said control switch is in an improper conduction state.
It will be appreciated by those skilled in the art that although the following Detailed Description will proceed with reference being made to preferred embodiments and methods of use, the present invention is not intended to be limited to these preferred embodiments and methods of use. Rather, the present invention is of broad scope and is intended to be limited only as set forth in the accompanying claims.